1. Field of the Invention
The present invention relates to an image reading apparatus for a contact image sensor (CIS) type and, more particularly, to a CIS-type image reading apparatus for reading an image of a light transmitting original such as a film.
2. Description of the Related Art
Mainly, there are widely-used image reading apparatuses for reading an original of: one using a CCD sensor and one using a contact image sensor unit.
In general, the image reading apparatus using the CCD sensor reads an image by employing a white light source, separating reflection light from an original into three colors of red (R), green (G), and blue (B) by a color filter, and detecting the color light by a sensor.
Meanwhile, in general, the image reading apparatus using the contact image sensor unit reads an image by switching and lighting on three-color LED light sources consisting of red (R), green (G), and blue (B) LED ones in the contact image sensor unit and by detecting reflection light from an original by a sensor. As compared with the image reading apparatus using the CCD sensor, usually, the image reading apparatus using the contact image sensor unit has a shorter length of an optical path, advantageously, equipment thereof can be thus reduced in size (be thinner) and troublesome optical adjustment is not needed. In place of the image reading apparatus using the CCD sensor, the image reading apparatus using the contact image sensor unit is widely used.
In the case of reading both a sheet original (reflecting sheet) and a light transmitting original such as a negative film, a positive film, or a slide, the image reading apparatus using the CCD sensor may comprise a white light source upstream of the original and can relatively easily read both the sheet original and the light transmitting original. Since a focusing depth is enough to read the light transmitting original, it is possible to set the light transmitting original on an original base without taking so care of the height from the original base made of a glass plate.
On the contrary, in the case of reading both the sheet original and the light transmitting original, the image reading apparatus using the contact image sensor unit requires light sources capable of sequentially switching three R-, G-, and B-colors upstream of the original. Advantageously, the image reading apparatus using the contact image sensor unit can be made thinner and can reduce consumption power as compared with one using the CCD sensor. However, the image reading apparatus using the contact image sensor unit has a focusing position of a lens, which is set on the glass-plate forming the original base, and has a short focusing depth of the lens and, therefore, the operation for reading the light transmitting original requires the accurate setting of the light transmitting original at the height position similar to that of the general sheet original.
FIG. 1 is a diagram showing one example of an image reading apparatus of a contact image sensor type for reading a light transmitting original. Referring to FIG. 1, the image reading apparatus of the contact image sensor type comprises a contact image sensor (CIS) unit 61 in a main body 62. A top surface of the main body 62 comprises an original base 63 made of a glass plate. The contact image sensor unit 61 is arranged in the proximity of the glass plate. A light transmitting original 64 is placed onto the original base 63. A area light source 65 is provided upstream of the original base 63, and comprises a light guide plate and LEDs for sequentially emitting three light of red (R)-, green (G)-, and blue (B). The area light source 65 is incorporated in an original cover (not shown) or is replaced with the original cover upon reading the light transmitting original 64.
Light outputted from the area light source 65 is transmitted to the light transmitting original 64 and is detected by a line sensor via a rod lens array in the contact image sensor unit 61, thus to read the light transmitting original 64.
As mentioned above, the image reading apparatus of the contact image sensor type has the focusing position of the lens onto the glass surface of the original base and the shorter focusing depth of the lens and, therefore, even when the original surface is slightly separated from the glass surface, the lens is not focused to the original surface.
Accordingly, by providing another area light source for reading the light transmitting original for the CIS-type image reading apparatus, the negative film or the positive film, as the light transmitting original, can directly be set onto the glass surface of the original base and be read with a preferable resolution.
However, when the negative film or the positive film, as the light transmitting original, has a holding frame such as a slide frame, a distance from the glass surface of the original base to the original surface varies depending on the presence or absence of the holding frame. Therefore, inconveniently, the lens is not focused to the original surface and the resolution deteriorates.
Differently from the image reading apparatus of the CCD sensor type, the image reading apparatus of the contact image sensor type for sequentially switching the three R-, G-, and B-LED light sources and reading the original cannot read the light transmitting original only by setting the white light source onto the original and lighting on it, and needs the arrangement at the top of the original, of a area light source comprising a light guide plate and R-, G-, B-LED light sources capable of being sequentially switched.
However, since the area light source comprising the light guide plate and the R-, G-, and B-LED light sources has an illuminance which is reduced as it exists more apart from the R-, G-, and B-LED light sources, it is not possible to manufacture the area light source having a uniform illuminance over a wide area. Consequently, the area light source comprising the light guide plate and the R-, G-, and B-LED light sources cannot read the light transmitting original having a wide area.
Further, as mentioned above, since the image reading apparatus of the contact image sensor type has the focusing position which is set onto the glass surface of the original base and has the short focusing depth, the reading of the light transmitting original requires the setting of the light transmitting original onto the glass surface of the original base similarly to the sheet original. However, when the light transmitting original such as a polyester-system film is placed onto the glass surface of the original base similarly to the sheet original, it is adhered to the glass surface, resulting in a problem in that a Newton ring is caused and an image to be captured has a poor quality.
In the image reading apparatus of the CCD sensor type, a readable wavelength is limited because the white light source is used. Thus, there is a problem in that it is not possible to read the light transmitting original and the sheet original having information capable of being read by light except for visible light, such as secret information or a discrimination mark which is not visible with the naked eye. Although the image reading apparatus of the CCD sensor type can be applied to a visible-light region, the operation for irradiating the light other than the visible light and reading the original needs the replacement of the light source for reading the original, using the light other than the visible light, every time.
Upon reading an original which mixedly includes the light transmitting original and the sheet original (e.g., reading a picture including a transparent region), currently, the image reading apparatus of the CCD sensor type has a problem in that it takes long time to read the original because transmission light and reflection light are separately read.